In identification systems based on pattern recognition, it is important that prominent image details and features be repeatably provided to the identification system. One method of providing the prominence of such details is the process of electrical enhancement of the electrical analog signal output of a recording camera. The electrical analog signal corresponds to light levels obtained from an image as it is scanned by the camera. The enhancement provides a greater pronunciation of light and dark levels, and such enhancement may be achieved by a conventional differentiating network consisting of a series capacitor and a shunt resistor.
It would be desirable to enhance the image details before they are recorded by the camera so that the analog signal better represents prominent details, thereby reducing the number of light-to-dark and dark-to-light variations to which post-camera enhancement circuits must respond, and in turn increasing the reliability of subsequent image or pattern recognition. For example, the palm of the human hand includes both friction ridges and creases; however, the crease details are far more prominent than the friction ridge details. Thus it would be desirable to present to the camera only the crease details.
When illuminated or flashed by visible incoherent light, the palm of the human hand reflects the light in such a manner that a camera cannot detect subtle pattern changes caused by irregular surface details such as ridges and creases. For this reason, mere illumination of the palm by visible light does not provide the desired enhancement, and, in fact, may even obscure certain details.
It is well known that most objects, when illuminated by light of a given wavelength, will either reflect or absorb the light. In many materials, the light emanating from the object is of a different wavelength than that of the incident light, and this phenomenon is known as luminescence. One type of luminescence is fluorescence, a radiative form of energy which occurs in accordance with the theory of quantum mechanics wherein an atom which has been excited by an external energy source relaxes from one state to another, giving up a quantum of energy in the form of light. Another type of luminescence that is caused by the absorption of radiations and continues for a noticeable time after the radiations have stopped is phosphorescence. Generally, the term fluorescence is used to indicate that the emitted radiation continues only as long as the excitation continues, and the term phosphorescence is used to indicate that the emitted radiation continues for some time after the excitation has ceased. It is generally recognized that the dividing line between these two types of luminescence is a decay time of 10.sup.-8 second.
Both fluorescence and phosphorescence can result from excitation by an ultraviolet light source, and depending upon the length of time that radiations therefrom are applied to a substance can excite the substance to a level of energy which will produce luminescence from some period of time until the energy level decays to a level which no longer emits a desired level of visible light. Fluorescent decay technology is utilized in laboratory research to aid in determining the presence of certain molecules in complex organic samples. One example of the use of phosphorescence is in radar indicator devices wherein latent target images decay slowly to permit tracking of aircraft.